Living organisms are mainly protected from foreign material by immune response. The immune system is comprised of a variety of cells and soluble factors produced therefrom. Among those cells, leukocytes, particularly lymphocytes, play a central role in the system. Lymphocytes are grouped into two major classes, B lymphocytes (hereinafter may be referred to as “B cells”) and T lymphocytes (hereinafter may be referred to as “T cells”). Cells of both classes can recognize antigens in a specific manner and interact with them to protect a living organism.
Immunotherapy has recently drawn attention as a forth method following surgery, chemotherapy and radiation therapy for treating tumors. Since immunotherapy utilizes the immunity inherent to humans, it is said that the physical burden on patients are less in immunotherapy than those in other therapies. The therapeutic approaches known as immunotherapies include: cell transfer therapy in which cells such as lymphokine-activated cells, natural killer T-cells or γδT cells obtained, for example, from exogenously-induced cytotoxic T-lymphocytes (CTLs) or peripheral blood lymphocytes by expansion culture using various method are transferred; dendritic cell-transfer therapy or peptide vaccine therapy by which in vivo induction of antigen-specific CTLs is expected; Th1 cell therapy; and immune gene therapy in which genes expected to have various effects are introduced ex vivo into the above-mentioned cells to transfer them in vivo. In these immunotherapies, CD4-positive T cells and CD8-positive T cells have traditionally known to play a critical role.
CD8-positive T cells are major effector cells that are capable of directly destroying tumor cells in vivo and in vitro. These cells are strictly specific to antigen peptides presented by MHC Class 1 molecules. In contrast, antigen specificities of NKT cells are not so strict, and they are considered to be effector cells that show intrinsic immune responses.
CD4-positive T cells are considered to have a fundamental role to regulate anti-tumor immune responses through a plurality of mechanisms although they do not destroy tumors directly. CD4-positive T cells that have recognized a tumor-antigen peptide represented by MHC Class II molecules promote the activation and proliferation of CTL through the interaction with antigen-presenting cells (APCs).
In contrast, CD25-positive/CD4-positive cells (regulatory T cells: Treg) have been shown to inhibit the anti-tumor immune responses and progression of various autoimmune diseases (see Patent Document 1 and Non-Patent Document 1). Specifically, since regulatory T cells suppress the activity of cytotoxic CD8-positive T cells through the control of the helper function by targeting CD4-positive T cells, some tumors are considered to utilize this system for their proliferation, thereby avoiding attack of the immune system.
GITR, which has been found as a gene expressed in regulatory T cells (see Non-Patent Document 1), is a cell surface transmembrane protein receptor and a member of the tumor necrosis factor receptor (TNFR) superfamily. GITR has been shown to be constitutively present on non-activated T cells. GITR binds to another transmembrane protein referred to as GITR ligand (hereinafter referred to as “GITRL”). Agonistic antibodies against GITR have been shown to abrogate the immunosuppressant activity of regulatory T cells, suggesting that GITRL plays a functional role in regulating the activity of regulatory T cells via GITR (see Non-Patent Document 2).